The invention generally relates to position sensors, and more particularly, to a magnetically sensitive sensor that detects a positional parameter of a rotating element.
Rotary sensors using contacting technology, such as potentiometers, suffer from various disadvantages that have limited their use. In applications requiring prolonged use over many years or requiring many cycles, the contacting sensors develop dead zones and non-uniform electrical behavior. Additionally, contacting sensors, even when in good condition, exhibit a relatively high degree of electrical noise during operation. This noise is a problem in sensitive electronic circuits.
It has been suggested to use non-contacting angular sensors to overcome the disadvantages of contacting sensors. Such sensors are not as susceptible to wear and exhibit reduced electrical noise. One barrier to the widespread use of non-contacting transducers, however, is the restricted angular range of those devices. Examples of such devices are shown in the U.S. Pat. Nos. 3,777,273; 3,988,710; 4,425,557 and 5,789,917. All of these devices have an angular operating range of less than one-half turn.
Another barrier to the widespread use of non-contacting sensors is the use of magnetic devices in such sensors. Magnetic fields are short acting, thus magnetic sensors have limited range and they exhibit undesirable signal-to-noise ratios (SNR) due to outside magnetic disturbances. The problems with non-linearity and SNR have been somewhat offset by the use of pole pieces, or flux directors. Flux directors attempt to extend the usable linear range of a magnetic field by advantageously shaping the field. The need to improve the SNR has been cited in various publications, including U.S. Pat. Nos. 5,444,369; 5,789,917; and 5,757,179.
There are contacting rotary sensors that exhibit extended range, but these sensors suffer from the disadvantages discussed above and are generally too complex and too costly for commercial acceptance. Moreover, these multi-turn sensors provide only a relative position indication, having no absolute reference.
The advantages of magnetic, non-contacting sensors over contacting potentiometric types include virtually unlimited operating life, owing to the fact that there are no physical contacts. Non-contacting sensors according to the principles of the invention do not suffer from the wear degradation and electrical noise exhibited by contacting sensors. A sensor according to the principles of the invention also offers a useful range of many full revolutions. Such a range makes the present invention a suitable replacement for absolute rotary encoders, angular sensors, potentiometers, tuners, and robotic joint sensors. A sensor according to the principles of the invention reduces SNR by converting a relatively large rotational mechanical input to a smaller linear mechanical translation of the magnet in the measurement circuit. This technique allows the sensor to use a very small portion of the magnetic field that is close to the magnet. This field portion offers the highest magnetic flux density for a given magnet and thus results in improved SNR. Also, the small size of this field portion reduces dependence on linearity.